The polymerization of monomers, particularly of styrene during processing is a matter of concern, because it causes formation of unwanted polymers and results in loss of yield of end product and makes the process un-economical.
In the prior art use of inhibitors and retarders, and combination thereof to overcome problem of polymerization of styrene has been reported.
The problem of using the inhibitors alone is that these are to be added continuously or at regular interval, because once they are consumed, the polymerization will re-start.
The problem of using the retarders alone is that these are not very effective to reduce polymerization of styrene to a level of substantial inhibition or to the commercially acceptable level of inhibition.
The prior art discloses use of some of quinone methide (QM) derivatives including 4-benzylidene, 2,6 di tert butyl cyclohexa-2,5 dienone as polymerization inhibitor. However, the inventor has found [refer to examples—Table 1] that main problem of using the quinone methide (QM) is that it has to be used in higher amounts to achieve commercially acceptable level of inhibition, and such higher amount not only results in increase of cost of process, but also results in formation of undesired products due to unstable nature of quinone methide.
The prior art also proposes quinone methide based composition comprising quinone methide (QM) derivatives including 4-benzylidene, 2,6 di tert butyl cyclohexa-2,5 dienone and 4HT (4 hydroxy tempo 2,2,6,6-tetramethyl-, 1-oxide) as styrene polymerization inhibitor. However, the inventor has found that main problem of using this known composition of quinone methide is that even at higher amounts the problem of polymerization is not resolved to commercially acceptable level.
The prior art [U.S. Pat. No. 7,651,635] discloses use of combination of an inhibitor and a retarder, wherein the inhibitor is “polymerization inhibitor” consisting of alkylhydroxylamine and retarder is 7-substituted quinone methide (substituted QM). The main problem of this composition is that it employs inhibitor, which are consumed continuously and get gradually depleted, and hence, the inhibitor has to be added continuously or intermittently or at least it has to be ensured that an appropriate amount of the inhibitor is maintained in the system [Col. 4, lines 7-13 of US'635].
The co-pending International (PCT) patent application no. PCT/IN2012/000553 discloses an additive composition comprising quinone methide (QM), and amine or oxide treated derivative of amine.
The another co-pending International (PCT) patent application no. PCT/IN2012/000839 discloses an additive composition comprising:
(A) one or more of the quinone methide or derivatives thereof,
(B) one or more of nitroxides (i.e. nitroxyl) compounds, and
characterized in that the said composition further comprises:
(C) one or more of aliphatic tertiary amines or mixture thereof.